insert sql consumes long time in Hibernate

Question

In my application I have two table, Group and Role, that are related between them with a @ManyToMany relationsheep. Since each Group can have many Roles and each Role can have many Groups.

The first one:

    @Table(name = "groups")
    @Cache(usage =  CacheConcurrencyStrategy.READ_WRITE)
    public class Groups {

 private Set<Role> roles = new HashSet<Role>();

    @ManyToMany( 
   fetch = FetchType.EAGER, 
   cascade = {CascadeType.DETACH, CascadeType.PERSIST, CascadeType.REFRESH})
   @JoinTable(name = "group_role", joinColumns = @JoinColumn(name = "group_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
 @IndexedEmbedded
 public Set<Role> getRoles() {
  return this.roles;  
 }
}

And the Role class is:

    @Entity
    @Table(name = "role")
    @Cache(usage =  CacheConcurrencyStrategy.READ_WRITE)
    public class Role {

 /** Field mapping. */ 
 private Set<Groups> groups = new HashSet<Groups>();

  @ManyToMany(
  fetch = FetchType.EAGER
  //,cascade = { CascadeType.PERSIST} 
  )
 @JoinTable(name = "group_role", joinColumns = @JoinColumn(name = "role_id"), inverseJoinColumns = @JoinColumn(name ="group_id" )) 
 public Set<Groups> getGroups() {
  return this.groups;  
 }
}

I want to add a role to a group, that means adding a record to the group_role table.

    @Transactional(readOnly = false, propagation = Propagation.REQUIRES_NEW)
 public void saveRoleToGroup(Group group,Role... roles) {
  groups.setRoles(roles);
  groupsDAO.merge(groups);
  LOG.debug("message2");
 }

It works and saves, just executing to the merge takes him 1 minutes!

watching the sql statment that are printed to the log, shows that after executing the follwing statment, there is a minute before closing the session.

15:55:56,875 DEBUG  - message2
15:55:56,906 DEBUG (SQLStatementLogger.java:111) - update groups set version=?, company_id=?, name=? where id=? and version=?
15:55:56,906 DEBUG (SQLStatementLogger.java:111) - insert into group_role (group_id, role_id) values (?, ?)
15:57:10,171 DEBUG     - end

The end statement comes from the calling method.

Why the simple insert statement consumes so much time (1 min)?